From seedling to flower head emergence (VE – R1)
Adequate moisture and soil temperature (>10*C), combined with the proper depth of sowing (3-5cm), constitute the most important factors for the fast and uniform growth of the crop.
At the early stages, plant growth is slow and it is mainly localized at their underground part. The plants develop a robust and deep root system, which allows them to utilize the water and nutrients in the deeper layers of the soil.
After the 8th true leaf has emerged, the plant enters the phase of vegetative growth and produces a rich leaf area. The flower head begins to form inside the plants, and the spaces where the flowers will develop gradually form on it. The head becomes visible once the plants have developed 14-16 leaves, while the flowers become fully differentiated and developed two weeks before flowering.
At the stage of fast growth, the plants absorb high quantities of nutrients and water in order to form a robust root system and rich foliage, which should support the formation of the head, flowering, and the filling of the seeds at later stages.
The nutrient absorption rate is low until the 4th leaf, increases between the 4th and 8th leaves, and is maximized after the 8th leaf, until the emergence of the head.
Nitrogen (Ν) contributes to the development of the leaf area, enhances photosynthesis, and increases the diameter of the inflorescence and the number of flowers formed on it, boosting the production potential.
Phosphorus (P) boosts the growth of the root system and the absorption of nutrients and water. It increases the number of formed flowers and improves flowering, thus increasing the production.
Potassium (Κ) favors the mechanical support of the frame of the plants, boosts root and leaf growth, enhances photosynthesis, regulates the water balance of the plants, and protects the crops from diseases and adverse weather conditions.
Regarding secondary elements, Sulphur (S) plays an important part in the utilization of Nitrogen and the development of the leaf area, while when it comes to Trace elements, Boron (Β) participates in the growth of young tissue and increases the production and fertility of pollen.
From the emergence of the head to the completion of flowering (R1 – R6)
This is the most crucial stage for crop growth and yields. First, immature inflorescence emerges at the top of the plants in the form of a small bud surrounded by a cluster of leaves.
The plants grow in height and continue to produce leaves up to the onset of flowering, and they complete the growth of the flowers, which reach reproductive maturation and are ready to be fertilized.
Flowering begins with the opening of the exterior immature ray flowers (yellow petals), gradually moves from the periphery to the center of the flower head, and is completed within 5-8 days with the wilting of the exterior flowers.
Throughout this period, from the emergence of the inflorescence to the completion of flowering, the plants have quite high requirements regarding nutrition and the environmental conditions. They absorb nutrients from the soil in order to form photosynthetic products, and at the same time they break down part of the substances they have stored in their leaves and stem in order to supply the head.
Adequate nutrients and water and the appropriate environmental conditions are a necessary condition for healthy flowering and successful pollination, and they have a determining effect on the final produce.
Nitrogen (Ν) increases the number of produced leaves and the production of photosynthetic products, and it boosts flowering and pollination.
Phosphorus (P) increases the number and fertility of the flowers and provides the plant with the necessary energy for flowering and pollination, as well as the structural elements for the development of seeds later on.
Potassium (Κ) is absorbed in large quantities and stored in the plant tissues, increasing the production and transfer of photosynthetic substances to the head and flowers.
Boron (Β) increases the production and fertility of pollen, thus improving flowering and fruit setting.
From the completion of flowering to ripening (R6 – R9)
With the completion of flowering, the development of the seeds and oil production begin. At a first stage, the hull of the seed starts growing, and it reaches its final size two weeks after flowering.
The development of the seed embryo begins on the 8th day after pollination with the accumulation of water, sugars, and proteins, followed by the formation of lipids at a later stage, which determines the oil concentration of the seeds.
About a month after flowering, the bracts surrounding the head turn brown from green and become brittle, while the base of the head turns yellow. These changes signify natural ripening, during which the filling of the seeds and production of oil are completed.
The dry weight of the seeds and their oil concentration and water reach their maximum value, and all that remains is for their high moisture (>30%) to gradually lessen so that they can be harvested a month later.
Throughout the period of the filling of the seeds and oil formation, in order to cover their increased needs, the plants transfer large quantities of the substances stored in the leaves to the growing seeds, while they continue to take up nutrients from the soil, particularly Nitrogen and Phosphorus.
Maintaining the foliage in a healthy condition, the presence of moisture and nutrients in the soil, and the appropriate temperature play an important part in achieving high yields in seeds and oil.
Nitrogen (Ν) continues to be absorbed in irrigated crops. It contributes to the preservation of the leaves and it increases the number and weight of the seeds. At this stage, excessive availability of Nitrogen has a negative effect on the plants, reducing the oil concentration of the seeds and delaying harvest.
Phosphorus (P), which continues to be taken up, and Potassium (Κ), which the plants had stored at previous stages, increase the size and weight of the seeds, improve its oil concentration, and enhance the quantity and quality characteristics of the production.